1. Field of the Invention
This invention relates to a method for adjusting conditions in a radiation image recording, read-out, and reproducing system wherein a radiation image is recorded on a recording medium, such as a stimulable phosphor sheet or X-ray film, the radiation image is then read out from the recording medium, an image signal representing the radiation image being thereby obtained, and a visible image is reproduced from the image signal.
2. Description of the Prior Art
Techniques for recording a radiation image on a recording medium, reading out the recorded radiation image in order to obtain an image signal, carrying out appropriate image processing on the image signal, and then reproducing a visible image by use of the processed image signal have heretofore been known in various fields. For example, as disclosed in Japanese Patent Publication No. 61(1986)-5193, an X-ray image is recorded on an X-ray film having a small gamma value chosen according to the type of image processing to be carried out, the X-ray image is read out from the X-ray film and converted into an electric signal (image signal), and the image signal is processed and then used for reproducing the X-ray image as a visible image on a copy photograph, or the like. In this manner, a visible image having good image quality with high contrast, high sharpness, high graininess, or the like can be reproduced.
Also, when certain kinds of phosphors are exposed to radiation such as X-rays, .alpha.-rays, .beta.-rays, .gamma.-rays, cathode rays or ultraviolet rays, they store part of the energy of the radiation. Then, when the phosphor which has been exposed to the radiation is exposed to stimulating rays such as visible light, light is emitted by the phosphor in proportion to the amount of energy stored thereon during its exposure to the radiation. A phosphor exhibiting such properties is referred to as a stimulable phosphor.
As disclosed in U.S. Pat. Nos. 4,258,264, 4,276,473, 4,315,318, 4,387,428, and Japanese Unexamined Patent Publication No. 56(1981)-11395, it has been proposed to use stimulable phosphors in radiation image recording and reproducing systems. Specifically, a sheet provided with a layer of the stimulable phosphor (hereinafter referred to as a stimulable phosphor sheet) is first exposed to radiation which has passed through an object, such as the human body. A radiation image of the object is thereby stored on the stimulable phosphor sheet. The stimulable phosphor sheet is then scanned with stimulating rays, such as a laser beam, which cause it to emit light in proportion to the amount of energy stored thereon during its exposure to the radiation. The light emitted by the stimulable phosphor sheet, upon stimulation thereof, is photoelectrically detected and converted into an electric image signal. The image signal is then used during the reproduction of the radiation image of the object as a visible image on a recording material such as photographic film, on a display device such as a cathode ray tube (CRT) display device, or the like.
Radiation image recording and reproducing systems which use stimulable phosphor sheets are advantageous over conventional radiography using silver halide photographic materials, in that images can be recorded even when the energy intensity of the radiation to which the stimulable phosphor sheet is exposed varies over a wide range. More specifically, since the amount of light which the stimulable phosphor sheet emits when being stimulated varies over a wide range and is proportional to the amount of energy stored thereon during its exposure to the radiation, it is possible to obtain an image having a desirable density regardless of the energy intensity of the radiation to which the stimulable phosphor sheet was exposed. In order to obtain the desired image density, an appropriate read-out gain is set when the emitted light is being detected and converted into an electric signal to be used in the reproduction of a visible image on a recording material, such as photographic film, or on a display device, such as a CRT display device.
In order ultimately to obtain a reproduced image having the best possible image quality, a novel radiation image recording and reproducing system has been proposed in, for example, U.S. Pat. No. 4,527,060. The proposed radiation image recording and reproducing system is constituted such that a preliminary read-out operation (hereinafter simply referred to as the "preliminary readout") is carried out. In the preliminary readout, a stimulable phosphor sheet on which a radiation image has been stored is exposed to a light beam having a comparatively low energy level, which releases part of the energy stored on the stimulable phosphor sheet when it was exposed to radiation. A preliminary read-out image signal is obtained from the preliminary readout. From the preliminary read-out image signal, information is ascertained about the radiation which was irradiated onto the stimulable phosphor sheet. Such information might indicate, for example, the energy intensity and dynamic range of the radiation. Thereafter, a final read-out operation (hereinafter simply referred to as the "final readout") is carried out. In the final readout, the stimulable phosphor sheet is exposed to a light beam having an energy level higher than the energy level of the light beam used in the preliminary readout. When the stimulable phosphor sheet is exposed to a light beam in the final readout, an image signal is obtained, which image signal is to be used in the reproduction of a visible image. Read-out conditions for the final readout and/or image processing conditions, under which the image signal obtained during the final readout is to be image processed, are adjusted automatically on the basis of the information ascertained from the preliminary read-out image signal.
The term "read-out conditions" as used herein means a group of various factors, which are adjustable and which affect the values of the image signal obtained during the final readout. The image signal in turn affects the gradation and sensitivity of the visible image which is reproduced. For example, the term "read-out conditions" may refer to a read-out gain, a scale factor, or the power of the source producing the light beam used during the final readout. Also, the term "image processing conditions" as used herein means a group of various factors which are adjustable and which affect how an image is processed, which in turn affects the gradation and sensitivity of the reproduced visible image. For example, the term "image processing conditions" may refer to the scale used in the conversion of an image signal.
The term "energy level of a light beam" as used herein means the level of energy of the light beam to which the stimulable phosphor sheet is exposed per unit area. In cases where the energy of the light emitted by the stimulable phosphor sheet depends on the wavelength of the irradiated light beam. i.e. the sensitivity of the stimulable phosphor sheet to the irradiated light beam depends upon the wavelength of the irradiated light beam, the term "energy level of a light beam" means the weighted energy level which is calculated by weighting the energy level of the light beam, to which the stimulable phosphor sheet is exposed per unit area, with the sensitivity of the stimulable phosphor sheet to the wavelength. In order to change the energy level of a light beam, light beams of different wavelengths may be used, the intensity of the light beam produced by a laser beam source or the like may be changed, or the intensity of the light beam may be changed by moving an ND filter or the like into and out of the optical path of the light beam. Alternatively, the diameter of the light beam may be changed in order to alter the scanning density, or the speed at which the stimulable phosphor sheet is scanned with the light beam may be changed.
Regardless of whether the preliminary readout is or is not carried out, it has also been proposed to analyze the image signal (or the preliminary read-out image signal) obtained and to adjust the image processing conditions, which are to be used when the image signal is processed, on the basis of the results of an analysis of the image signal. The proposed method is applicable to cases where an image signal is obtained from a radiation image recorded on a recording medium such as conventional X-ray film, as well as to systems using stimulable phosphor sheets.
In the aforesaid radiation image recording and reproducing systems, the read-out conditions for the final readout and/or the image processing conditions are adjusted such that each reproduced visible image has the best possible quality. Therefore, with the aforesaid radiation image recording and reproducing systems, each reproduced image is suitable for viewing. However, in cases where different read-out conditions for the final readout and/or different image processing conditions are used when an image is reproduced, a change in the image density in, for example, an image of an object which was recorded in the past and an image which represents the current state of the same object cannot be ascertained accurately when the two images are compared with each other.
An example of the aforesaid problem will be described hereinbelow with reference to FIGS. 3A, 3B, 4A, and 4B.
FIG. 3A is a schematic view showing a radiation image 6 of part (in this case, the frontal chest) of a human body, which image has been reproduced on a sheet of photographic film 5. A portion 6b in the radiation image 6 represents a part of the chest not affected by disease and has an approximately uniform level of image density, whereas a portion 6a represents a part of the chest affected by disease and has a level of image density lower than the image density at the portion 6b.
FIG. 3B is a graph showing the change in the abnormal level of image density at the portion 6a representing a part of the chest affected by disease, which change was investigated by recording and reproducing a plurality of radiation images of the object shown in FIG. 3A over a period of time. The disease is becoming less severe as time passes. Curve A indicates the ideal levels of image density in the reproduced image of the portion 6a as the disease becomes less severe over time. However, in cases where the image readout and/or the image processing is carried out such that each of the reproduced images has the best possible quality, abnormal levels of image density in the reproduced image of the portion 6a are often represented by curve B. The severity of the disease is judged on the basis of the difference between the image density of the part of the reproduced image corresponding to the portion 6a and the image density of the part of the reproduced image corresponding to the portion 6b. Specifically, from an image which was recorded at time T1, the image density difference .DELTA.D1 between the normal level of image density and the level of image density on curve A should be detected. The image density difference .DELTA.D1 is large and indicates that the part of the chest represented by the portion 6a has been severely affected by disease. However, there is a risk that the image density difference .DELTA.D1' between the normal level of image density and the level of image density on curve B will be detected. The image density difference .DELTA.D1' is small and indicates that the disease is not very severe. Also, from an image which was recorded at time T2, the image density difference .DELTA.D2 (.DELTA.D2&lt;.DELTA.D1) between the normal level of image density and the level of image density on curve B should be detected. However, there is a risk that the image density difference .DELTA.D2' (.DELTA.D2'&gt;.DELTA.D1') between the normal level of image density and the level of image density on curve B will be detected. At time T2, the disease has actually become somewhat less severe than it was at time T1, i.e. .DELTA.D2&lt;.DELTA.D1. However, there is a risk that the disease will be judged as being more serious than it was at time T1, i.e. .DELTA.D2'&gt;.DELTA.D1'.
FIG. 4A is a schematic view showing a radiation image 6' of part (in this case, the sides of the vertebrae) of a human body, which image has been reproduced on a sheet of photographic film 5'. In cases where a disease is diagnosed which causes the level of image density at bone portions 6a', 6a', . . . in the radiation image 6' to increase, a judgment about the progression of the disease must be made on the basis of the image density at the bone portions 6a', 6a', . . . This is because, unlike the case shown in FIG. 3A, the bone portions 6a', 6a', . . . and a portion 6b' are images of different organs, and the difference in image density therebetween cannot be utilized to make a judgment.
However, when a plurality of reproduced images such as those shown in FIG. 4A are obtained, the read-out conditions for the final readout and/or the image processing conditions are generally adjusted so that the image density of the bone portions 6a', 6a', . . . is constant in the reproduced images.
FIG. 4B is a graph showing the change in the abnormal levels of image density at the bone portions 6a', 6a', . . . , which change was investigated by recording and reproducing a plurality of radiation images such as those shown in FIG. 4A over a period of time. As in the case shown in FIG. 3B, the disease is becoming less severe with the passage of time. Curve A' indicates the ideal levels of image density in the reproduced images of the bone portions 6a', 6a', . . . as the disease becomes less severe. However, in cases where the image readout and/or the image processing is carried out such that the levels of the image density in the reproduced images are identical at the bone portions 6a', 6a', . . . , the image density at the bone portions 6a', 6a', . . . follows curve B' and is approximately the same as the level of image density in a reproduced image of bone portions not affected by disease. In this case, any abnormality in the image density in the reproduced images of the bone portions 6a', 6a', . . . cannot be detected.
In order for the aforesaid problems to be eliminated, a novel method has been proposed in U.S. Pat. No. 4,999,497. With the proposed method, when each of a plurality of radiation images of an object is recorded, an image of a step wedge comprising a plurality of steps whose transmittances with respect to radiation vary stepwise is recorded together with the image of the object. When a radiation image, which is to be compared with a radiation image recorded in the past, is currently read out, processed, and reproduced as a visible image, the read-out conditions for the final readout and/or the image processing conditions are adjusted such that the image density of the part of a currently reproduced image corresponding to each step of the step wedge may become identical with the image density of the part of a past reproduced image corresponding to the corresponding step of the step wedge. However, with this proposed method, it is necessary for the same step wedge to be always recorded together with the object. Therefore, considerable time and labor are required to manage the step wedge, and the image recording efficiency cannot be kept high. Also, when the read-out conditions for the final readout and/or the image processing conditions are adjusted on the basis of the image signal (or the preliminary read-out image signal) detected from a radiation image, image signal components of the image signal corresponding to the image of the step wedge must be extracted from the image signal. Thereafter, the read-out conditions for the final readout and/or the image processing conditions must be adjusted on the basis of the values of the image signal components corresponding to each step of the step wedge. Therefore, complicated operations are required to adjust the read-out conditions for the final readout and/or the image processing conditions.